Field of the Invention
The present invention relates to an image sensor and an image capturing apparatus.
Description of the Related Art
Recently, there has been developed a CMOS image sensor which has a plurality of pixels arranged in row and column directions, with analog/digital converters (A/D converters) being arranged by column of a plurality of pixels, and is designed to convert analog signals from pixels into digital signals and output them. When outputting a pixel signal by column from the image sensor by A/D-converting the signal using the A/D converter arranged for each column, it is possible to digitally perform horizontal transfer of the pixel signals from the image sensor.
The technique disclosed in Japanese Patent Laid-Open No. 2005-278135 uses a single-slope type A/D converter as an A/D converter. A single-slope type A/D converter includes a comparator and a counter. While an analog pixel signal is input to one input terminal of the comparator, a ramp signal whose voltage level changes with a predetermined slope with the lapse of time is input as a reference signal to the other input terminal. An output from the comparator is inverted when the magnitude relation between the pixel signal and the reference signal is inverted. The counter starts counting when a ramp signal is generated. When an output from the comparator is inverted, the counter stops. The value counted by the counter becomes a digital value corresponding to the level of the analog pixel signal.
In addition, the technique disclosed in Japanese Patent Laid-Open No. 2005-278135 uses, as a means for taking the difference between the reset component of a pixel signal and the signal component generated by a photodiode, a method of subtracting the reset component from the signal component by inverting the counting direction of an UP/DOWN counter. This method requires only one counter, and does not require any subtractor which subtracts a reset component from a signal component, thereby achieving a reduction in circuit scale.
Each pixel of a recent CMOS image sensor has a feature that two or more photodiodes share a readout circuit after a floating diffusion which transfers the signal generated by a photodiode. According to Japanese Patent Laid-Open No. 2004-134867, when signals are output from two photodiodes sharing a floating diffusion, a reset level is read out first, and the signal of the first photodiode is then read out. In addition, while the floating diffusion is not reset, the signal of the second photodiode is read out following the signal of the first photodiode. The respective signals are subtracted to read out signals obtained by excluding the reset level from each of the signals from the two photodiodes. This method can save the time taken to read out one reset level, and hence can shorten the time required to read out pixel signals.
In addition, as disclosed in Japanese Patent Laid-Open No. 2001-83407, there is available a technique of arranging photodiodes for autofocus for a CMOS image sensor. According to Japanese Patent Laid-Open No. 2001-83407, two photodiodes are provided below one microlens. The respective photodiodes are configured to receive light from different pupil planes of an image capturing lens. Comparing outputs from the two photodiodes can perform focus detection. It is also possible to use the sum of outputs from the two photodiodes as an image signal.
However, the single-slope type A/D converter using the UP/DOWN counter disclosed in Japanese Patent Laid-Open No. 2005-278135 cannot use the technique of shortening the time like that disclosed in Japanese Patent Laid-Open No. 2004-134867. According to the method disclosed in Japanese Patent Laid-Open No. 2005-278135, when a reset level is read out first and then the signal of the first photodiode is read out, the counter is inverted to obtain a signal by subtracting the reset level from the signal of the first photodiode. As a result, the information of the reset level is lost. When the signal of the second photodiode is A/D-converted following the signal of the first photodiode, the value of the signal of the first photodiode and the value of reset-level signal are also lost. This makes it impossible to perform proper difference processing. It is therefore difficult to use the technique disclosed in Japanese Patent Laid-Open Nos. 2005-278135 and 2004-134867 to shorten the time taken to read out signals in a CMOS image sensor having two photodiodes provided for one microlens as disclosed in Japanese Patent Laid-Open No. 2001-83407.